Optical disc

ABSTRACT

An optical disc comprising transparent substrate, a phase-changing material layer disposed on the transparent substrate and made of a material which is phase-changeable by radiation of a light beam, a reflecting film layer disposed on the phase-changing material layer, and a cooling layer disposed between the phase-changing material layer and the reflecting layer, wherein the cooling layer has a heat conductivity ranging from 0.25 J/cmKs to 20.00 J/cmKs. The optical disc of the present invention has such a structure that a crystal phase portion of the phase-changing material layer shows a higher light absorptivity than that of the amorphous phase portion and exhibits a high carrier to noise ratio (C/N ratio) irrespective of a line speed upon rotational driving of the optical disc.

This is a continuation of application Ser. No. 08/564,640 filed Nov. 29,1995, now U.S. Pat. No. 5,657,310.

BACKGROUND OF THE INVENTION

This invention relates to an optical disc on which information isrecorded, or from which the information recorded is reproduced, byradiation of a light beam from a laser, and more particularly to anoptical disc suitable for recording information at a high density.

An optical disc of a so-called phase-changing type is known in the art.The optical disc of this type includes a transparent substrate on whichat least a phase-changing material layer, a protective layer and areflecting layer are formed. Further, the reflecting layer is coveredwith an protective over-coating layer, if necessary.

In the optical disc of such a type, the phase-changing material layer isinitially maintained in a crystalline state. When a write beam from alaser is radiated on a predetermined spot-like portion of a surface ofthe optical disc, the portion of the phase-changing material layer isallowed to change from a crystal phase to a liquid phase. Thereafter,the liquid phase portion of the phase-changing material layer is rapidlycooled to form an amorphous phase portion in the form of pit. As aresult, the amorphous phase portion of the phase-changing material layerformed by radiation of the write beam from the laser is caused to have adifferent reflection coefficient from the inherent crystal phase portionthereof. A read-out (reproduction) of the information recorded isperformed by detecting a signal indicative of change (reduction) in thereflection coefficient of the amorphous phase portion.

Meanwhile, in the case of the optical disc of such a phase-changingtype, the recording has been generally carried out by using a pitposition-recording method.

In addition to this recording method, there has been also known aso-called pit length-recording method capable of recording aninformation at a high density. However, the pit length-recording methodhas such a problem that the recording accuracy and therefore the exactreproduction of data recorded is likely to be adversely affected due topossible fluctuation of the position of the amorphous pit formed on thesurface of the optical disc. Consequently, in the case where therecording of information signal on the optical disc is carried out byusing the pit length-recording method, it is desired that the layerstructure on the substrate is so formed that the fluctuation of the pitposition is unlikely to occur.

In the known optical disc, in order to prevent occurrence of thefluctuation of the pit position, the inherent crystal phase portion ofthe phase-changing material layer has a higher light absorptivity, i.e.,a higher thermal absorptivity than that of the amorphous phase portionthereof which is formed by radiation of the light beam. However, in theoptical disc having such a construction, the phase-changing materiallayer can be cooled only at a slow rate due to the high absorptivity ofthe crystal phase portion, so that there occurs a problem that thewriting of the information signal must be performed at a line speed of10 m/second or greater to avoid too much retention of heat in thecrystal phase portion. The present invention has been made to overcomethe above-mentioned problems encountered in the prior art.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anoptical disc having such a layer structure capable of being rapidlycooled nevertheless the crystal phase portion of the phase-changingmaterial layer shows a higher light absorptivity, i.e., a higher thermalabsorptivity than that of the amorphous phase portion thereof. Theoptical disc of the present invention exhibits a suitable carrier tonoise ratio (hereinafter referred to as a "C/N ratio") even though thewriting of the information signal on the optical disc can be performedat a line speed of 10 m/second or lower.

It has been found by the present inventors that, when a protective andcooling layer or a cooling layer having a heat conductivity in apredetermined range is provided between a phase-changing material layerand a reflecting film layer and when the protective and cooling layer orthe cooling layer is formed of a material exhibiting a transparency in arange of the wave-length of a visible light, namely having nointerference with the visible light, the layer structure of the opticaldisc can be rapidly cooled nevertheless the crystal phase portion of thephase-changing layer shows a higher light absorptivity, i.e., a higherthermal absorptivity than that of the amorphous phase portion thereof.

In accordance with the present invention, there is provided an opticaldisc comprising a transparent substrate, a phase-changing material layerdisposed on the transparent substrate and made of a material which isphase-changeable by radiation of a light beam, a reflecting film layerdisposed on the phase-changing material layer, and a cooling layerdisposed between the phase-changing material layer and the reflectingfilm layer, wherein the cooling layer has a heat conductivity rangingfrom 0.25 J/cmKs to 20.00 J/cmKs.

The optical disc according to the present invention is of aphase-changing type in which a portion of the phase-changing materiallayer is changeable from a crystal phase to a liquid phase when a writebeam from a laser is radiated on a surface of the optical disc and thenthe liquid phase portion formed by the radiation of the write beam israpidly cooled to form an amorphous phase portion in the form of pit. Byadopting the above-mentioned layer construction, the rapid cooling ofthe liquid phase portion of the phase-changing material layer can beachieved nevertheless the crystal phase portion shows a higher lightabsorptivity, i.e., a higher thermal absorptivity than that of theamorphous phase portion.

In the optical disc according to the present invention, a protectivelayer may be formed separately from the cooling layer. In this case, theprotective layer and the cooling layer is in turn disposed between thephase-changing material layer and the reflecting film layer.

Alternatively, a single layer having a combined function of theprotective and cooling layers may be formed between the phase-changingmaterial layer and the reflecting layer.

The combined protective and cooling layer or the single cooling layerthus formed between the phase-changing material layer and the reflectinglayer on the transparent substrate, has a heat conductivity of thelimited range and is formed of a material exhibiting a transparency in arange of the wave length of a visible light, namely having nointerference with the visible light so that the phase-changing materiallayer can have a rapidly-cooled structure. Even though the crystal phaseportion of the phase-changing material layer shows a higher lightabsorptivity, i.e., a higher thermal absorptivity than the amorphousphase portion thereof, the output having a high C/N ratio can beobtained irrespective of the line speed upon the writing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an optical disc according to oneembodiment of the present invention.

FIG. 2 is a sectional view of an optical disc according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown an optical disc according to a firstembodiment of the present invention. The optical disc comprises atransparent substrate 1 on which a groove 7 is formed. Formed on thetransparent substrate are in turn a phase-changing material layer 2, aprotective and cooling layer 3 having combined protective and coolingfunctions and a reflecting film layer 6.

The transparent substrate 1 may be formed, for example, of a materialsuch as an acrylic resin, a polyolefin resin, glass or the like.

First, the phase-changing material layer 2 is formed on the transparentsubstrate 1. The phase-changing material layer 2 initially shows acrystal phase structure before radiation of a write beam. When thephase-changing material layer is exposed to the write beam, for examplea light beam emitted from a laser, the portion radiated by the writebeam undergoes a phase change from a crystal phase to a liquid phase.Thereafter, the liquid phase portion is rapidly cooled so as to undergoa further phase change from a liquid phase to an amorphous phase so thatthe portion radiated by the write beam has a different reflectioncoefficient from those portions which are not radiated by the write beamand therefore remains in a state of a crystal phase.

Examples of such a phase-changing material layer 2 may include As-Te-Geseries film layer, Te-Ge-Sn series film layer, Te-Ge-Sn-O series filmlayer, Te-Se series film layer, Sn-Te-Se series film layer, Te-Ge-Sn-Auseries film layer, Ge-Sb-Te series film layer, Sb-Te-Se series filmlayer, In-Se-Tl series film layer, In-Sb series film layer, In-Sb-Seseries film layer, Ag-Zn alloy film layer, Cu-Al-Ni alloy film layer,In-Se-Tl-Co series film layer, Si-Te-Sn series film layer, a thin filmlayer of a low-oxide such as TeO_(x) wherein X represents a numbergreater than 0 but less than 2, or the like.

A method of forming the phase-changing material layer is notparticularly restricted but any known method such as a depositing methodand a spattering method can be employed.

Formed on the phase-changing material layer 2 is the protective andcooling layer 3 having a combined protective and cooling functions. Theprotective and cooling layer 3 may be formed of a material having a heatconductivity ranging from 0.25 J/cmKs to 20.00 J/cmKs and exhibiting atransparency in a range of a visible light, namely having nointerference with the visible light. Examples of the material usable forthe protective and cooling layer 3 may include a nitride, an oxide, asulfide, a carbide or other compounds of metal or a semiconductorelement selected from Al, Zn, Zr, Si, or the like.

Generally, as is known, the material which has no interference with avisible light having a wave length λ of 4000 Å or greater (4000 Å is awave-length of a light located in a boundary region between a visiblelight and an ultraviolet light), includes those having an energy gay Egof 3 eV or more. Specific examples of suitable material used for theprotective and cooling layer 3 may include Al₂ O₃, AlN, Al₂ S₃, ZnS,ZnO₂, SiO₂, Si₃ N₄, SiC, SiS₂, or the like.

Further, a reflecting film layer 6 is formed on the protective andcooling layer 3. Examples of a suitable material used for the reflectingfilm layer 6 may include, for example, Dy, Al, Au or the like.

Referring to FIG. 2, an optical disc according to a second embodiment ofthe present invention is shown in section. In contrast with the opticaldisc of the above first embodiment as shown in FIG. 1 in which thesingle layer 3 having a combined protective and cooling functions isprovided between the phase-changing material layer 2 and the reflectingfilm layer 6, the optical disc of the second embodiment as shown in FIG.2 has two separate protective layers in addition to the protective andcooling layer 3. The first protective layer 4 is disposed between thetransparent substrate 1 and the phase-changing material layer 2 whilethe second protective layer 5 is disposed between the phase-changingmaterial layer 2 and the protective and cooling layer 3.

The first and second protective layers 4 and 5 may be formed of, forexample, a nitride, an oxide, a sulfide or other compounds of metal or asemiconductor element such as Al, Si and the like, which do not show anabsorptivity in a range of the wave-length of a light beam emitted froma semiconductor laser.

Moreover, a further protective layer (not shown) may be formed on thereflecting film layer 8, if desired.

EXAMPLES:

The present invention is now described in more detail by way of examplesbut these examples are not intended to-constitute a limitation of thepresent invention.

Example 1

First, a glass 2P substrate was used so as to obtain an transparentsubstrate 1 for an optical disc on which a groove 7 having a track pitchP of 1.0 μm, a depth d of about 80 nm and a width W of 0.35 μm wasformed. Incidentally, the symbol "2P" means that the glass substrate ismanufactured by a photo-polymerization method.

A composition composed of 80 parts of ZnS and 20 parts of SiO₂ wasdeposited onto a primary surface of the transparent substrate 1 to forma first protective layer 4 having a thickness of 100 nm. Further, aternary alloy having a formula of Ge₀.22 Sb₀.22 Te₀.66 (wherein thenumber appearing on the lower right side of each element indicates aratio thereof) was deposited on the first protective layer 4 to form aphase-changing material layer 2 having a thickness of 20 nm.

Successively, a composition composed of 80 parts of ZnS and 20 parts ofSiO₂ was deposited onto the phase-changing material layer 2 to form asecond protective layer 5 having a thickness of 5 nm.

Furthermore, aluminum nitride (AlN) was deposited on the secondprotective layer 5 to form a protective and cooling layer 3 having afilm thickness of 45 nm. Then, a gold (Au) was deposited on theprotective and cooling layer 3 to form a reflecting film layer 6 havinga thickness of 10 nm whereby an optical disc of the present inventionwas obtained.

The thus obtained optical disc was radiated by a write beam emitted froma laser to write an information signal thereon. In this case, thewriting was performed under such conditions in which a line speed forrotational driving of the optical disc was set to 5 m/second, and otherparameters were set to the following values; a wave length of a lightbeam emitted from a laser diode to 680 nm, a numerical aperture of anobjective lens to 0.55, a frequency for driving the laser diode ordata-writing to 3.75 MHz, a pulse width to 107 ns, a write laser powerto 15 mW and a read laser power to 1 mW. After the written informationsignal was erased at an erasing laser power of 7 mW, the optical discwas radiated by a light beam from the laser in which the same conditionsas described above was employed except that the frequency was changed to1.4 MHz, to write another information signal on the optical disc.

As a result, it was recognized that an erasing ratio of the previouslyrecorded information signal was 35 dB, which shows that the optical dischad a good erasing performance.

Example 2

An optical disc was produced in the same manner as described in Example1 above except that the second protective layer 5 composed of 80 partsof ZnS and 20 parts of SiO₂ was omitted and the protective and coolinglayer 3 having a film thickness of 50 nm was directly laminated over thephase-changing material layer 2 composed of AlN.

The thus obtained optical disc was subjected to writing and erasing ofthe information signals under the same conditions and in the same manneras described in Example 1. The optical disc was further radiated by alight beam at a frequency of 1.4 MHz to write another information signalthereon.

As a result, it was confirmed that an erasing ratio of the previouslyrecorded information signal was 30 dB which also shows that the opticaldisc had a good erasing performance.

Comparative Example 1:

Example 1 was repeated in the same manner as described above except thatthe thickness of the second protective layer 5 composed of 80 parts ofZnS and 20 parts of SiO₂ was changed to 50 nm and the protective andcooling layer 3 composed of AlN was omitted.

The thus obtained optical disc was subjected to writing and erasing ofthe information signals under the same conditions and in the same manneras described in Example 1. The optical disc was further radiated by alight beam at a frequency of 1.4 MHz to write another information signalthereon.

As a result, it was confirmed that an erasing ratio of the previouslyrecorded information signal was 10 dB. This shows that the optical dischad a poor and insufficient erasing performance because a good erasingratio for digitally recorded signals is generally 20 dB or more.

As is apparent from the above description, in accordance with thepresent invention, a protective and cooling layer or a cooling layer,which has a heat conductivity falling within the above-mentionedpredetermined range and exhibits a transparency in a range of the wavelength of a visible light, namely has no interference with the visiblelight, is formed between the phase-changing material layer and thereflecting layer, so that the phase-changing material layer of theoptical disc has a structure capable of being rapidly cooled down. Thelayer structure effectively prevents occurrence of fluctuation of thepit positions on the optical disc nevertheless the crystal phase portionof the phase-changing material layer shows a higher light absorptivity,i.e., a higher thermal absorptivity than the amorphous phase portionthereof.

What is claimed is:
 1. An optical disc comprising:a transparentsubstrate; a phase-changing material layer disposed of said transparentsubstrate and made of a material which is phase-changeable by radiationof a light beam, said phase-changing material selected from the groupconsisting of As-Te-Ge series films, Te-Ge-Sn series films, Te-Ge-Sn-Oseries films, Te-Se series films, Sn-Te-Se series films, Te-Ge-Sn-Auseries films, Ge-Sb-Te series films, Sb-Te-Se series films, In-Se-Tlseries films, In-Sb series films, In-Sb-Se series films, Ag-Zn alloyfilms, Cu-Al-Ni alloy films, In-Se-TI-Co series films, Si-Te-Sn seriesfilms, a thin film layer of TeO_(x) wherein x represents a numbergreater than 0 but less than 2, a portion of said phase-changingmaterial layer being changeable to a liquid phase when the portion isradiated by the light beam and then the liquid phase portion is rapidlycooled to form an amorphous phase portion which has a differentreflection coefficient from that of an inherent crystal phase of saidphase-changing material layer; a reflecting film layer disposed on saidphase-changing material layer; a cooling layer disposed between saidphase-changing material layer and said reflecting layer; and furthercomprising a protective layer which is interposed between saidphase-changing material layer and said cooling layer; wherein, saidcooling layer is formed of Al₂ O₃, AlN, Al₂ S₃, ZnS, ZnO₂, SiO₂, Si₃ N₄,SiC or SIS₂, said cooling layer has a heat conductivity ranging from0.25 J/cmKs to 20.00 J/cmKs and an energy gap of 3 eV or greater; aportion of said phase-changing material layer is changeable to a liquidphase when the portion is radiated by a write beam and then the liquidphase portion is rapidly cooled to form an amorphous phase portion whichhas a different reflection coefficient from that of an inherent crystalphase of said phase-changing material layer, and said cooling layerfunctions as a protective layer.